Field of the Invention
Embodiments of the present invention relate to a mixing apparatus for mixing fluid components, such as the mixing of phosgene and amine in a reactive chemical process.
Description of the Related Art
The field of conventional mixing devices can be roughly divided into two main areas: dynamic or mechanical mixers and static mixers. Dynamic or mechanical mixers rely on some type of moving part or parts to ensure the desired or thorough mixing of the components. Static mixers generally have no prominent moving parts and instead rely on flow profiles and pressure differentials within the fluids being mixed to facilitate mixing. The current disclosure is mostly directed to a static mixer but could also be used in combination with dynamic mixers.
Isocyanates are molecules characterized by N═C═O functional groups. The most widely used isocyanates are aromatic compounds. Two aromatic isocyanates are widely produced commercially, namely, toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). Isocyanates may be reacted with polyols to form polyurethanes. Major polyurethane applications are rigid foams, which are good insulators and are heavily used in appliance, automotive and construction businesses; and flexible foams which may be used in mattresses and furniture applications. In addition aliphatic isocyanates such as hexamethylene diisocyanates are also widely produced and used in special applications such as abrasion and UV resistant coatings.
Mixing is important in isocyanate production. The isocyanate product quality and yield are dependent on a multistep chemical reaction network. In the first step of the process, two continuous streams of reactants (amine and phosgene) are mixed. Secondary reactions like the reaction between phosgenation products and amine to form ureas and other urea derivatives ultimately reduce the quality of the product composition. While the production of isocyanates is desired, secondary reactions lead to the creation of undesired products. Some of these secondary reactions are believed to create products such as ureas and urea derivatives like carbodiimides, and uretonimines. The overall chemical reaction can be depicted as follows:
Amine+Phosgene→Isocyanate+Hydrochloric Acid+Ureas+Carbodiimides+Uretonimines+Undesired products
While many known and unknown factors control the quality of the principal reaction, an increase of the ratio of phosgene to amine, a dilution of amine in a solvent, or an improved mixing minimizes the formation of undesired by-products. Some of the undesired byproducts may be solids and may be associated with fouling in process equipment.
Consequently, mixer designs with improper mixing can result in lower overall yield of the desired product or can generate a product that clogs or fouls the reactor system leading to down time and/or increased maintenance costs.
FIG. 1 schematically illustrates phosgene concentration within a static mixer of the prior art. FIG. 1 illustrates a partial sectional view of a cylindrical conduit 3 where a phosgene flow 1 goes from the left to the right and an amine flow 2 is injected into the phosgene flow 1 from a jet 4 formed through the cylindrical conduit 3. As amine traverses and reacts with the phosgene, principal and secondary reactions occur. A circle 5, which is located at the distance L where amine flow 1 enters, illustrates a region on the downstream side of the jet 4 where the phosgene concentration is relatively low (near zero). Because phosgene and amine reactions are exothermic, the regions surrounding circle 5 have increased temperature. The low concentration of phosgene and increased temperature promote the formation of undesirable secondary reactions and production of by-product.
It would be desirable to have a static mixer that improves phosgene and amine mixing thus limiting the production of undesired by-products.